轉自：
http://blog.csdn.net/lingedeng/article/details/6996599
nsdate類提供了建立date，比較date以及計算兩個date之間間隔的功能。date對象是不可改變的。
如果你要建立date對象并表示目前日期，你可以alloc一個nsdate對象并調用init初始化：
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nsdate *now = [[nsdate alloc] init];
或者使用nsdate的date類方法來建立一個日期對象。如果你需要與目前日期不同的日期，你可以使用nsdate的initwithtimeinterval...或datewithtimeinterval...方法，你也可以使用更複雜的calendar或date
components對象。
建立一定時間間隔的nsdate對象：
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nstimeinterval secondsperday = 24 * 60 * 60;
nsdate *tomorrow = [[nsdate alloc] initwithtimeintervalsincenow:secondsperday];
nsdate *yesterday = [[nsdate alloc] initwithtimeintervalsincenow:-secondsperday];
[tomorrow release];
[yesterday release];
使用增加時間間隔的方式來生成nsdate對象：
nsdate *today = [[nsdate alloc] init];
nsdate *tomorrow, *yesterday;
tomorrow = [today datebyaddingtimeinterval: secondsperday];
yesterday = [today datebyaddingtimeinterval: -secondsperday];
[today release];
如果要對nsdate對象進行比較，可以使用isequaltodate:, compare:, laterdate:和 earlierdate:方法。這些方法都進行精确比較，也就是說這些方法會一直精确比較到nsdate對象中秒一級。例如，你可能比較兩個日期，如果他們之間的間隔在一分鐘之内則認為這兩個日期是相等的。在這種情況下使用，timeintervalsincedate:方法來對兩個日期進行比較。下面的代碼進行了示例：
if (fabs([date2 timeintervalsincedate:date1]) < 60) ...
月曆對象封裝了對系統日期的計算，包括這一年開始，總天數以及劃分。你将使用月曆對象對絕對日期與date components（包括年，月，日，時，分，秒）進行轉換。
nscalendar定義了不同的月曆，包括佛教曆，格裡高利曆等（這些都與系統提供的本地化設定相關）。nscalendar與nsdatecomponents對象緊密相關。
你可以通過nscalendar對象的currentcalendar方法來獲得目前系統使用者設定的月曆。
nscalendar *currentcalendar = [nscalendar currentcalendar];
nscalendar *japanesecalendar = [[nscalendar alloc] initwithcalendaridentifier:nsjapanesecalendar];
nscalendar *userscalendar = [[nslocale currentlocale] objectforkey:nslocalecalendar];
userscalendar和currentcalendar對象是相等的，盡管他們是不同的對象。
你可以使用nsdatecomponents對象來表示一個日期對象的元件——例如年，月，日和小時。如果要使一個nsdatecomponents對象有意義，你必須将其與一個月曆對象相關聯。下面的代碼示例了如何建立一個nsdatecomponents對象：
nsdatecomponents *components = [[nsdatecomponents alloc] init];
[components setday:6];
[components setmonth:5];
[components setyear:2004];
nsinteger weekday = [components weekday]; // undefined (== nsundefineddatecomponent)
要将一個日期對象解析到相應的date components，你可以使用nscalendar的components:fromdate:方法。此外日期本身，你需要指定nsdatecomponents對象傳回元件。
nsdate *today = [nsdate date];
nscalendar *gregorian = [[nscalendar alloc] initwithcalendaridentifier:nsgregoriancalendar];
nsdatecomponents *weekdaycomponents = [gregorian components:(nsdaycalendarunit | nsweekdaycalendarunit) fromdate:today];
nsinteger day = [weekdaycomponents day];
nsinteger weekday = [weekdaycomponents weekday];
同樣你也可以從nsdatecomponents對象來建立nsdate對象：
[components setweekday:2]; // monday
[components setweekdayordinal:1]; // the first monday in the month
[components setmonth:5]; // may
[components setyear:2008];
nsdate *date = [gregorian datefromcomponents:components];
為了保證正确的行為，您必須確定使用的元件在月曆上是有意義的。指定“出界”月曆元件，如一個-6或2月30日在公曆中的日期值産生未定義的行為。
你也可以建立一個不帶年份的nsdate對象，這樣的作業系統會自動生成一個年份，但在後面的代碼中不會使用其自動生成的年份。
[components setmonth:11];
[components setday:7];
nsdate *birthday = [gregorian datefromcomponents:components];
下面的示例顯示了如何從一個月曆置換到另一個月曆：
nsdatecomponents *comps = [[nsdatecomponents alloc] init];
[comps setday:6];
[comps setmonth:5];
[comps setyear:2004];
nsdate *date = [gregorian datefromcomponents:comps];
[comps release];
[gregorian release];
nscalendar *hebrew = [[nscalendar alloc] initwithcalendaridentifier:nshebrewcalendar];
nsuinteger unitflags = nsdaycalendarunit | nsmonthcalendarunit | nsyearcalendarunit;
nsdatecomponents *components = [hebrew components:unitflags fromdate:date];
nsinteger day = [components day]; // 15
nsinteger month = [components month]; // 9
nsinteger year = [components year]; // 5764
在目前時間加上一個半小時：
nsdatecomponents *offsetcomponents = [[nsdatecomponents alloc] init];
[offsetcomponents sethour:1];
[offsetcomponents setminute:30];
// calculate when, according to tom lehrer, world war iii will end
nsdate *endofworldwar3 = [gregorian datebyaddingcomponents:offsetcomponents todate:today options:0];
獲得目前星期中的星期天（使用格裡高利曆）：
// get the weekday component of the current date
nsdatecomponents *weekdaycomponents = [gregorian components:nsweekdaycalendarunit fromdate:today];
/*
create a date components to represent the number of days to subtract from the current date.
the weekday value for sunday in the gregorian calendar is 1, so subtract 1 from the number of days to subtract from the date in question. (if today is sunday, subtract 0 days.)
*/
nsdatecomponents *componentstosubtract = [[nsdatecomponents alloc] init];
[componentstosubtract setday: 0 - ([weekdaycomponents weekday] - 1)];
nsdate *beginningofweek = [gregorian datebyaddingcomponents:componentstosubtract todate:today options:0];
optional step:
beginningofweek now has the same hour, minute, and second as the original date (today).
to normalize to midnight, extract the year, month, and day components and create a new date from those components.
nsdatecomponents *components = [gregorian components:(nsyearcalendarunit | nsmonthcalendarunit | nsdaycalendarunit) fromdate: beginningofweek];
beginningofweek = [gregorian datefromcomponents:components];
如何可以計算出一周的第一天（根據系統的月曆設定）：
nsdate *beginningofweek = nil;
bool ok = [gregorian rangeofunit:nsweekcalendarunit startdate:&beginningofweek interval:null fordate: today];
獲得兩個日期之間的間隔:
nsdate *startdate = ...;
nsdate *enddate = ...;
nsuinteger unitflags = nsmonthcalendarunit | nsdaycalendarunit;
nsdatecomponents *components = [gregorian components:unitflags fromdate:startdate todate:enddate options:0];
nsinteger months = [components month];
nsinteger days = [components day];
使用category來計算同一時代（ad|bc）兩個日期午夜之間的天數：
@implementation nscalendar (myspecialcalculations)
-(nsinteger)dayswithinerafromdate:(nsdate *) startdate todate:(nsdate *) enddate {
nsinteger startday=[self ordinalityofunit:nsdaycalendarunit inunit: nseracalendarunit fordate:startdate];
nsinteger endday=[self ordinalityofunit:nsdaycalendarunit inunit: nseracalendarunit fordate:enddate];
return endday-startday;
}
@end
使用category來計算不同時代（ad|bc）兩個日期的天數：
@implementation nscalendar (myothermethod)
-(nsinteger) daysfromdate:(nsdate *) startdate todate:(nsdate *) enddate {
nscalendarunit units=nseracalendarunit | nsyearcalendarunit | nsmonthcalendarunit | nsdaycalendarunit;
nsdatecomponents *comp1=[self components:units fromdate:startdate];
nsdatecomponents *comp2=[self components:units fromdate enddate];
[comp1 sethour:12];
[comp2 sethour:12];
nsdate *date1=[self datefromcomponents: comp1];
nsdate *date2=[self datefromcomponents: comp2];
return [[self components:nsdaycalendarunit fromdate:date1 todate:date2 options:0] day];
判斷一個日期是否在目前一周内（使用格裡高利曆）：
-(bool)isdatethisweek:(nsdate *)date {
nsdate *start;
nstimeinterval extends;
nscalendar *cal=[nscalendar autoupdatingcurrentcalendar];
nsdate *today=[nsdate date];
bool success= [cal rangeofunit:nsweekcalendarunit startdate:&start interval: &extends fordate:today];
if(!success)
return no;
nstimeinterval dateinsecs = [date timeintervalsincereferencedate];
nstimeinterval daystartinsecs= [start timeintervalsincereferencedate];
if(dateinsecs > daystartinsecs && dateinsecs < (daystartinsecs+extends)){
return yes;
}
else {
return no;
}
方法2：
+(nsstring*)getydealdatetostring:(int)day{
nsdate *curdate = [nsdate date];
nscalendar* calendar = [nscalendar currentcalendar];
nsdatecomponents* comps = [calendar components:nsyearcalendarunit|nsmonthcalendarunit|nsweekcalendarunit|nsweekdaycalendarunit fromdate:curdate];
comps = [calendar components:nsyearcalendarunit|nsmonthcalendarunit|nsweekcalendarunit|nsweekdaycalendarunit fromdate:curdate];
[comps setmonth:[comps month]+1];
[comps setday:day];
nsdate *tdatemonth = [calendar datefromcomponents:comps];
nsstring *datestr=[nsstring stringwithformat:@"%@",tdatemonth];
return datestr;
}
參數：day天數，可以是負數
用法：
nslog(@"%@",[你的類名 getydealdatetostring:-30]);
如果今天的日期是2013-03-31 21：56：23
則輸出：2013-02-28 21：56：23
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